Analysis of Satellite and in-situ Optical Proxies for PIC and POC during
GEOTRACES GP15 and GP17-OCE Transects from the Subarctic North Pacific
to the Southern Ocean
Abstract
The biological pump, a fundamental process governing atmospheric CO2,
rapidly transfers particulate inorganic and organic carbon (PIC and POC)
from surface waters to the deep sea but is inherently highly variable in
space and time, and thus poorly observed. Here we synthesize PIC and POC
data from satellites, CTD-profiled optical sensors (birefringence and
transmissometer), and from in-situ pumps samples from GEOTRACES
transects spanning 20,000 km from the North Pacific to Southern Ocean.
High resolution profile data from PIC sensors revealed strong subsurface
maxima in the deepest euphotic zone waters of oligotrophic gyres;
furthermore, data showed high concentrations of PIC penetrating to
>500 m south of the Subarctic Front (45°N – 35°N), at the
equator, and north of the Antarctic Polar Front (45°S – 55°S)
indicating high carbon export in these regions. We developed a new
interpolation scheme for satellite data that improved matchups with ship
observations. North of the Antarctic Polar Front (APF), PIC sensor data
was generally well aligned with sample PIC; however, a positive bias of
satellite PIC was found in poor retrieval regions. South of the APF,
both satellite and birefringence sensor greatly overestimated PIC by
factors of >25 and 12, respectively, compared to sample PIC
which averaged 15nM. The unanticipated discovery of a strong
non-carbonate particle birefringence source coupled with a microscopic
investigation of pump samples leads us hypothesize that internal
reflection within bubbles and/or body structures of heavily silicified
colony-forming diatoms (Fragilariopsis and Pseudo-nitzschia) is the
cause for both anomalous birefringence and backscattered satellite
radiances.